Heretofore, the major purpose of air filtration was to reduce the density of dust, consisting mostly of 0.2 micron, or larger, particles from air. Recent environmental concerns in the living, industrial and military environs have expanded the desired scope of air filtration into the "suppression of odor and virus/gems." Yet current air purification methods are hindered in many ways by an inability to capture undesirable submicron particles, microorganisms, odors, and substances efficiently and economically without suffering from high pressure drop, short life, energy inefficiency, and poor reliability. Simply stated, there is no current adequate method to meet the needs of modern air purification.
There have been thousands of experiments, research efforts, and patents made in the field of air filtration. However, all of this work has followed three existing basic principles: (1) the mechanical filter (mechanical blocking of airborne particles by mesh) (a few hundred years old--but sill the most common and widely used method); (2) the electrostatic precipitator (invented 90 years ago in 1906 by Cottrell, which relies on ionization and the Coulomb's law attraction of particle separation for filtration); and (3) the precharged synthetic fiber filter (precharged fibers create an electrostatic field within the filter material and interact with and capture airborne particles).
A summary of these three known principles, or methods, is presented below:
In the mechanical filtration method, substantial improvements took place, such as the utilization of cellulose material, synthetic fibers (HEPA & ULPA filters), etc.; but the basic form is still the mechanical blocking of particles. This method has been adequate for the filtration of particles larger than 0.2 microns. However, filtration of submicron particles (odor, virus, etc.) through porous material (less than 0.05 microns) creates a tremendous amount of pressure drop, energy inefficiency, and high cost of operation, and therefore is not practical. Currently, such needs are supplemented by additional costly devices, e.g. activated charcoal deodorizers, and the like.
Many types of electrostatic filters have been proposed for removing small particulate materials such as dust, smoke, viruses, and the like from gases such as air or the exhaust gases of vehicles or industrial processes. Typically, such filters rely in one way or another on the ionization of the particulate material by a fixed high voltage electric field, so that they may be trapped and held by electrostatic forces. Common disadvantages of ionizing electrostatic filters are that they operate at sufficiently high voltages, requiring expensive insulation and safety precautions, as well as substantial electric power, and that they produce ozone, which constitutes a health hazard.
There are a number of additional problems with known electrostatic filter technologies, whereby the attraction and collection of particulates to the filter materials are accomplished by Coulomb's Law, including flocculating effects, creating unpredictable occasional bursts of release of dust, inadequate dust-holding capacity, requiring more frequent maintenance, and other common disadvantages associated with high voltage utilization. Thus, electrostatic filtration is used today mainly as a pre-filter or general purpose filter for commercial purposes, without requiring realistic high performance.
Starting in the 1970's, precharged synthetic material fibers were introduced. Typically, a square or rectangular cross section synthetic fiber is utilized to retain electrostatic charge within the filter material. This is a hybrid of the mechanical and electrostatic filtration methods. The main problem is internal discharge in a relatively short time, resulting in degradation of the filtration capability.
Turning now to a history of the present invention, in 1991, Y. Yamamoto discovered a new method whereby a properly set non-ionizing electrical field could create a random, high speed, and churning motion of airborne particles in perpendicular directions to the air flow through a filter medium placed in the electric field, see U.S. Pat. No. 5,368,635. The said churning motion inside of a filter medium dramatically increases the probability of particles to bombard the surfaces of the fibers which compose the filter medium. Thus, a combination of such motion and Van der Waals force interaction between particles and fiber surfaces tremendously increases the probability of capturing particles throughout the filter material. This method efficiently captures a wide range of particles, in fact, even submicron particles which are much, much smaller than the porosity of the filter medium used.
This invention simply but dramatically upgrades the efficiency of filtration of most of the filter media known today, including paper, glass fiber, synthetic fiber, cloth, natural fiber, foam and electrostatically charged materials. This method provides the advantages of 1) high efficiency of filtration, 2) capturing particles in a wide range of sizes, including below submicron sizes, 3) the least amount of pressure drop, 4) improvement in energy efficiency, and 5) low cost. These advantages are achieved without any change in the mechanical properties of the filter.